This invention relates to a novel method for machining a workpiece with a beam of radiant energy; for example, a laser beam, in the presence of and assisted by a chemically-reactive gas. The term "machining" includes drilling, cutting, scoring and any other procedure which alters the shape of the workpiece by removal of material from the workpiece due to the action of the beam.
Many complex processes interact when a workpiece is machined with a beam of radiant energy by a prior method. The beam first heats the incident area of the surface of the workpiece to melting. Then, molten workpiece material is further heated to the vaporization (or decomposition) temperature of the material. As this happens, and particularly after boiling begins, considerable amounts of gas can be evolved, and this can lead to appreciable gas pressures, which tend to eject the molten material from deeper portions of the workpiece, leading to "splats," buildup of material around the incident area, and/or the scattering of energy of the incident laser beam in the "plasma plume" of vaporized material. All these processes are generally undesirable and can lead to irregular hole shapes. Also, vaporized material tends to condense around the edge of the machined portion of the workpiece.
By raising the intensity of the incident beam, the amount of liquid material generated before vaporization begins is reduced, leading to generally "cleaner" holes or cuts, but greatly reduced machining efficiency. Sometimes the machining speed is greatly reduced due to laser-beam absorption in the very high temperature plasma plume produced, when the incident intensity approaches the 10.sup.7 -10.sup.8 W/cm.sup.2 range.
Plastics and other insulators are generally easier to machine than metals because their lower thermal diffusivities result in less molten material being generated in a given time interval for a given incident intensity; their absorptions of radiant energy are generally higher, leading to intrinsically higher absorbed power densities for given laser powers; and they often have no real fluid stage, passing from an extremely viscous liquid state to the vapor state. Metals, on the other hand, melt and become quite fluid at temperatures well below their vaporization point.
One can convert laser machining processes in metals to ones much more like those in plastics by using chemical reactions between the beam-heated metal and a surrounding gas. In a sense, this has already been done, by using an air (or oxygen) jet coaxially with the laser beam to cut through metals. Here the energy produced by the oxidation reaction is used to reduce the vaporization energy requirement put on the laser beam. However, most metal oxides have high melting points and very low volatilities at the oxidation-reaction temperatures. As a result, considerable energy must be employed to remove the oxide material, or else it comes off in irregular droplets that lead to irregularly cut edges.